The present invention relates to semiconductor devices and more particularly to III-nitride semiconductor devices and methods of fabricating III-nitride semiconductor devices.
A III-V semiconductor is a semiconductor material that is composed of a group III element and a group V element. III-V semiconductors are desirable for power applications, but have not been exploited extensively due in part to difficulties in fabrication.
For example, one commercially desirable III-V semiconductor is III-nitride. Note that as used herein III-nitride semiconductor or GaN-based semiconductor refers to a semiconductor alloy from the InAlGaN system. Examples of alloys from the InAlGaN system include GaN, AlGaN, AlN, InN, InGaN, and InAlGaN. Note that while nitrogen is present in each alloy, the presence and proportion of In, Al, or Ga can be varied to obtain an alloy in the InAlGaN system.
III-nitride semiconductor devices are desirable for power applications due in large part to the high band gap of III-nitride semiconductor materials. To fabricate a III-nitride semiconductor device at least one III-nitride semiconductor alloy (i.e. an alloy from the InAlGaN system) needs to be formed over a substrate. The three well known substrate materials for III-nitride semiconductor devices are sapphire, SiC and Si.
Silicon substrates are more desirable commercially because of low cost, and high thermal conductivity. However, due to lattice mismatch and differences in the thermal expansion characteristics of III-nitride semiconductor alloys and silicon, thick III-nitride semiconductor layers (e.g. more than 1 micron thick) either crack or cause the silicon wafer to bend. It should be noted that the cracking problem associated with thick III-nitride semiconductor layers is not experienced only when a silicon substrate is used, and thus the problem is not limited to III-nitride semiconductor that is formed on silicon substrates.
To overcome the cracking problem a transition layer is disposed between the active portion of the device and the substrate. Referring thus to FIG. 1, a known III-nitride semiconductor device includes an active semiconductor region 10 formed on transition layer 12, which is formed over substrate 14. Substrate 14 is, for example, a silicon diode.
Active region 20 includes a first III-nitride semiconductor body 16 of one band gap, and a second III-nitride semiconductor body 18 of another band gap forming a heterojunction with first III-nitride semiconductor body 16. A two dimensional electron gas (2DEG) is formed at the heterojunction of first III-nitride semiconductor body 16 and second III-nitride semiconductor body 18 through which current is conducted between first power electrode 20 (e.g. source electrode) and second power electrode 22 (e.g. drain electrode) both electrically coupled to second III-nitride semiconductor body 18. As is well known, application of a proper voltage to gate electrode 24 can disrupt, or restore 2DEG in order to control the current between first power electrode 20 and second power electrode 22.
In order to obtain the best possible control over the current between first power electrode 20, and second power electrode 22, it is desirable to ensure that current cannot find any alternative path but through the 2DEG. It has, however, been observed that current can find a leakage path through transition layer 12 and trough substrate 14, when substrate 14 is electrically conductive.
It is desirable to reduce or eliminate the leakage paths through transition layer 12 in order to improve the switching characteristics of a III-nitride power semiconductor device.